Tuesday, 27 August 2013

PostgreSQL, Binary replication in practice

 A couple of days ago I started making a short howto about streaming replication in PostgreSQL 9.2. Most of these things are well documented but in this howto i will also try to experiment with switch overs and switchbacks. It aims to show how easy it is to set it up right out of the box.

Streaming replication PostgreSQL 9.2

For my example i will use 2 debian VM's 192.168.0.100 (pglab1) and 192.168.0.101 (pglab2)


- not mandatory -

exchange ssh keys for passwordless ssh ,might be used if we need to scp scripts , rsync or whatever.

if you don't know how to do it, follow these steps :
http://www.debian-administration.org/articles/152

- ON MASTER (PGLAB1)  -
after you create a postgres cluster using initdb,
edit master's postgresql.conf and change the following :
listen_addresses = '*'
wal_level = hot_standby #(could be archive too)
max_wal_senders = 5
hot_standby = on


create a replication user :
create user repuser replication password 'passwd';

edit pg_hba.conf and add :


host all all 192.168.0.0/0 trust
host replication repuser 192.168.0.0/0 md5


- ON SLAVE -

Now ssh to slave and from $PGDATA run :

pg_basebackup -D /opt/data/ -v -h 192.168.0.100 -U repuser
 enter password , this will transfer a full copy of your cluster from your master. check the documentation of pg_basebackup for compression and other options available.

In $PGDATA edit a file called recovery.conf containing :
standby_mode = on
 primary_conninfo = 'host=192.168.0.100 port=5432 user=repuser password=passwd'


with the master up, start slave , it should say :
LOG: database system is ready to accept read only connections


At this point you will have 2 nodes running with your master accepting read/write operations and your slave accepting only read only operations (reporting goes here maybe ?) now, lets say the master crashes and you need to failover and promote slave as the new master.

-FAILOVER-

shutdown master 
on slave, execute:
pg_ctl promote


that's it, your slave (pglab2) is now master accepting all kinds of connections.

now lets say that the ex-master (pglab1) , is fixed and is ready to come up again,

- Switch back to original Master -


on the current master (pglab2) :

echo "select pg_start_backup('clone',true);" |psql pgbench
rsync -av --exclude postgresql.pid /opt/data/* 192.168.0.100:/opt/data/
echo "select pg_stop_backup();"|psql pgbench

this will sync all data from my current master (pglab2) to my current -to be- slave (pglab1), should be currently down.

edit recovery.done and fix the ip of the current master
rename recovery.done to recovery.conf
start ex-master (pglab1), now as slave , promote it with the slave (pglab2) down,
recreate slave (pglab2) with rsync, edit recovery.conf and start it again, 
the servers now have their original roles.


note that you can have 2 master databases but this might (and probably will) create a mess, so be sure that you bring down the correct server before promoting.

if you want to check if a server is acting as master or slave , run :

select pg_is_in_recovery()
If it's true, you're on a slave.


Thanks for reading.

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